This invention relates to a cooling air pick up device for use in a gas turbine engine.
In a gas turbine engine it is common practice to cool engine components exposed to very high temperature gas flows such as the turbine blades and turbine stator vanes. This cooling generally uses air to cool the components either by transferring heat energy from the component to air passing through it or shielding the components from the hot gas by generating a thin barrier layer of cooling air across the surface of the component. Often the two techniques are combined by cooling air passing through a component and then being ejected from the component through a number of apertures to generate a film or barrier layer of air over the component surface.
In order for film cooling to be useful it is clearly necessary for the pressure of the cooling air to be higher than the ambient pressure in the hot gas flow around the component. Additionally it is convenient to exhaust expended cooling air into the gas flow through the engine rather than provide further ducting to carry it away. This also requires that cooling air pressure is higher than the stream pressure in the hot gas flow around the component.
Generally cooling air is tapped off from the compressed airflow exiting the compressor or compressors because this is the highest pressure cool air available within the engine. This tapping off is carried out by a cooling air pick up.
It has been found in some engine designs that higher pressure cooling air is desirable. This can be obtained by further pressurising the cooling air using a dedicated compressor driven by the gas turbine engine. This is undesirable because it increases the cost, weight and complexity of the engine, and adds a serious safety problem should this compressor fail. Vital engine parts will then at best be uncooled and at worst have hot gas flowing into and through them due to pressure driven reverse flow in the cooling air system. Either event will usually result in the rapid destruction of the component. Additionally there will be some power losses involved in driving such a compressor, reducing the net power output of the gas turbine engine.
Additionally, in a multi-compressor engine where the compressors are arranged in series it is desirable to obtain cooling air from the output of the earliest possible compressor in the series in order to minimise the amount of work done by the engine compressing its cooling air. However, it is generally necessary to take cooling air for the guide vanes and turbine blades immediately downstream of the combustion region from the output of the last compressor in the series because of the relatively high gas pressure in this region.